In the macroscopic world, one can measure the width of a feature, for example a line, by measuring the distance between two opposing edges of that feature. One might expect that by providing a suitably powerful microscope, one could measure the width of a very small feature in much the same way. In fact, this is not the case. As a feature's width approaches the wavelength of the light used to observe it, the edge appears to lose its sharpness. Instead of a sharp edge, one observes, through the microscope, a gradual transition from a region that is clearly on one side of the edge to another region that is clearly on the other side of the edge.
No amount of focusing can correct the blurriness of the edge under these circumstances. Nor can the microscope somehow be made “better” in any meaningful way. The loss of a sharp edge arises not from some defect in the microscope but from the wave nature of light itself.
In the manufacture of integrated circuits, one often uses photolithographic masks to form features on a substrate. In an effort to pack more circuit elements into the integrated circuits, the critical dimensions of these features are made on the order of a wavelength of visible light. The masks used to make integrated circuits often have corresponding features whose critical dimensions are likewise on the order of an optical wavelength. To ensure the quality of these masks, these dimensions are verified with an optical microscopy. However, because of their small size, these features appear, in such a microscope, to have blurred edges.